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Problems with 416SS after e-polish

We're experiencing an interesting problem with our 416SS components. We machine 416 rounds for an optical application that have 2-56 and 4-40 blind holes on the top and bottom. We machine, stress relieve, hone the ID, grind for flatness, then electropolish the part for uniformity. Before electropolishing we are placing the parts in our sonic washer for approx. 15 minutes to loosen any particles that may be trapped in the tapped holes. After that we wash the parts in hot water and soap, rinsing them off and blowing out the tapped holes to ensure nothing is seeping out during e-polish. The problem is we are getting excessive staining on the parts and what appears to be corrosion or rust in the tapped holes. Our plater doesn't seem to have any answers as to why this could be happening. Could the process we are using in washing these parts be effecting the e-polishing? We do a tremendous amount of work with 416SS and e-polish all of it but only seem to have this problem with parts with blind holes. Any suggestions?

Michael Damon
Manufacturer - Rochester, New York, USA
November 9, 2010

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There are a number of things that could be going on here. As you may know, 416 is a high sulfur grade, so what you THINK may be rust coming out of the blind holes could be sulfides. You can test for this in a number of ways, but a quick test would be to immerse the "rusty" parts in a 5% solution of NaOH overnight. If the "rust" disappears it is probably sulfides.

Of course, the epolish process that you are using is probably not getting into the blind holes, so you have surfaces in the holes that are not polished. This would make them much more susceptible to corrosion and rusting. Since 416 is not a very corrosion resistant steel, this could be a problem.

Lastly, you may consider passivation of the parts, making sure that the passivation medium gets INTO the blind holes to remove any free iron that is left on the surface. Ultrasonics can help to make this happen.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

First of two simultaneous responses -- November 19, 2010

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Michael, my first thought was that the 416 was causing the problem. My experience has been that the manganese sulfides that make it a free machining stainless steel (think "controlled amounts of dirt") can lead to corrosion and staining problems during passivation and in service. Yet you're saying that it's only in the blind holes that the problem is occurring, and that suggests a couple of things to me. First is that something funky is happening with the current densities or the electropolish chemistry within that small volume within the blind hole, where I suspect you DON'T have a conforming anode to try to keep uniform current. That suggests that conforming anodes or nozzles aimed at the holes to get more bath circulation there might help the situation. The other thing I wonder about is whether you need to electropolish the blind holes- it may be that the simple soap & water wash and blow dry would be enough for your product. If so, I'd go with the simple work-around and just plug those blind holes with rubber plugs during the electropolish.

Good luck to you!

Lee Gearhart
metallurgist - E. Aurora, New York
Second of two simultaneous responses -- November 21, 2010